The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The engine controls the air-fuel mixture to achieve an ideal air-fuel mixture ratio (stoichiometric ratio). At the optimum stoichiometric ratio, all of the fuel is burned using all of the oxygen in the air. For internal combustion engines, the stoichiometric ratio is about 14.7:1. In other words, for each pound of gasoline, 14.7 pounds of air is burned. The air-fuel mixture varies from the optimum stoichiometric ratio during driving. Sometimes the air-fuel mixture is lean (an air-to-fuel mixture higher than 14.7) and other times the air-fuel mixture is rich (an air-to-fuel mixture lower than 14.7).
Vehicle engines produce oxides of nitrogen (NOx) as a component of vehicle emissions. In particular, lean-burn gasoline and diesel engines tend to produce higher levels of NOx than conventional stoichiometric gasoline engines.
In an effort to reduce NOx levels in vehicle emissions, manufacturers employ emissions control systems with engine sensors and NOx storage catalysts, sometimes referred to as Lean NOx traps (LNTs). The NOx storage catalysts absorb and decompose the NOx with combustible gases such as carbon monoxide (CO) or hydrocarbon (HC). While reducing NOx levels, these systems tend to increase the level of hydrocarbons in vehicle emissions.
LNTs require periodic intervals of rich exhaust gas to regenerate the stored NOx and convert it into harmless byproducts. This control of the air-fuel ratio in a diesel engine can cause torque disturbance during rich operation.